The present invention relates to a brake system for a vehicle, and more particularly to a brake system with a supplemental braking system which intensifies the braking forces to ensure the operation of brakes particularly when the braking forces are insufficient.
As for a vehicle such as an automobile, a wheel (wheels) is sometimes locked when braking the vehicle, thereby rendering the driving unsteady and extending the stopping distance. Therefore, conventionally, anti-skid brake systems (hereinafter, sometimes referred to as "ABS") of various types for giving an anti-skid brake control (hereinafter, sometimes referred to as "ABS control") when a braked wheel is locked have been proposed. According to the ABS control, when a tendency toward wheel lock is detected during braking, the tendency toward wheel lock is canceled by decreasing the braking forces and then by intensifying the braking forces again, with the result that the driving is stabilized and the stopping distance is minimized.
FIG. 4 is a view showing a brake system having ABSs of return pump type as an example of the conventional ABS. This brake system 1 is a dual circuit hydraulic brake actuating system in which front and rear wheels F, R are independent, and comprises ABSs 2 each provided for each braking circuit. Like parts of the respective brakes are given by like reference numerals.
In FIG. 4, the numeral 3 designates a brake pedal, 4 designates a tandem master cylinder (hereinafter, sometimes referred to as "MCY"), the numeral 5 designates a reservoir of the master cylinder 4, the numeral 6 designates each ABS control holding valve which is a normally open solenoid valve in which a communicating position I and an interrupting position II are set, the numeral 7 designates each wheel cylinder (hereinafter, sometimes referred to as W/C) which communicates with the master cylinder 4 through a brake fluid line, the numeral 8 designates each ABS control pressure-reducing valve which is a normally closed solenoid valve in which an interrupting position II and a communicating position II are set, the numeral 9 designates each sump which is a low pressure accumulator, the numeral 10 designates each ABS control pump (hereinafter, sometimes referred to "pump" only), the numeral 11 designates each ABS control modulator comprising the ABS control holding valve 6, the ABS control pressure-reducing valve 8, the sump 9, and the pump 10, and the numeral 12 designates a proportioning valve (hereinafter, sometimes referred to as "PV") for the rear wheel for reducing the upward gradient of the brake pressure when exceeding a predetermined value.
Though the brake system 1 is shown only for the front left wheel FL and the rear left wheel RL, the front right wheel FR and the rear right wheel RR are provided with the respective ABS control modulators 11 in the same manner.
In the fluid pressure brake system 1 having such conventional ABSs 2, when no force is exerted on the brake pedal 3, the pumps 10 are not actuated so as not to work and the ABS control holding valves 6 and the ABS control pressure-reducing valves 8 are set at the positions I as shown in the drawing. Therefore, the W/Cs 7 of the front and rear wheels F, R communicates with the reservoir 5 of the MCY 4 and no W/C pressure Pw in the W/Cs 7 i.e. no fluid pressure is produced.
Upon pedaling the brake pedal 3 for normal braking, the MCY 4 produces MCY fluid pressure Pm which is transferred to the respective W/Cs 7 through the ABS control holding valves 6 in the front wheel brakes and through the ABS control holding valves 6 and PVs 12 in the rear wheel brakes, thereby braking all of the front and rear wheels F, R.
Upon releasing the brake pedal 3 to cancel the braking, the MCY 4 does not work so as not to produce the MCY fluid pressure Pm, thereby canceling the braking on the front and rear wheels F, R.
When it is detected that at least one of the front and rear wheels F, R is in locking tendency, an electronic control unit, not shown, sets the ABS control holding valves 6 in the ABS control modulators 11 of the front and rear wheels F, R at the interrupting positions II to hold the brake fluid pressures in the W/Cs 7 at the brake fluid pressure at this point to prevent the tendency toward wheel lock from glowing. When it is detected that at least one of wheels is still in the locking tendency even after holding the brake fluid pressure, the electronic control unit sets the ABS control pressure-reducing valve 8 corresponding to that wheel at the communicating position II to discharge the brake fluid of the corresponding W/C 7 to the sump 9 in order to reduce the brake fluid pressure of the W/C 7 and to actuate the corresponding pump 10.
Upon reducing the brake fluid pressure, the tendency toward wheel lock is canceled and the rotation of the wheel is restored to some extent. Then, the electronic control unit sets the ABS control pressure-reducing valve 8 at the interrupting position I and sets the ABS control holding valve 6 at the communicating position I. Therefore, fluid pressure Pp from the pump 10 as well as the MCY fluid pressure Pm is supplied to W/C 7 to increase the brake fluid pressure of the W/C 7 again.
The electronic control unit controls the ABS control holding valves 6, the ABS control pressure-reducing valves 8, and the pumps 10. In this manner, the ABS control is performed by holding, reducing, and increasing the brake fluid pressures until completely canceling the tendency toward wheel lock.
In the brake system 1 with such ABSs 2, for example when there is a high probability that the vehicle comes into collision and an emergency braking is needed, a driver e.g. a beginner who is unpracticed in driving can not pedal the brake pedal 3 with all his/her might in such a manner as to produce the maximum braking force. Therefore, the braking forces are insufficient and braking forces needed for emergency braking are not given, so that it is impossible to ensure the emergency braking. In a brake system provided with a vacuum brake booster or a fluid-pressure brake booster, there is a probability that braking forces needed for emergency braking are not given when the pressure produced by the pressure source of the brake booster is reduced for some reason.
Therefore, it is desirable that the brake system 1 is provided with a supplemental braking system which intensifies the braking forces to ensure the braking particularly in such a case that the braking forces are insufficient. It is also desirable that the brake system is made as simply as possible to restrict the increase in cost when the supplemental braking system is incorporated in the brake system which has a fluid pressure control system such as ABS 2.